1. Field of the Invention
The present invention relates to an antenna device having an antenna and a parallel feeder on a dielectric plate.
2. Description of the Related Art
A planar small antenna device of the type having an antenna such as a dipole antenna on/in a dielectric plate made of synthetic resin or glass is known in the art. An example of this type of antenna device is shown in FIG. 9. The antenna device of FIG. 9 has a dipole antenna 12 formed of a conductor pattern on a dielectric plate 11. The dipole antenna 12 is connected through a coaxial cable 13 to a connector 14 fixed to an end of the dielectric plate 11. The connector 14 is connected to a radio transmitter/receiver, so radio communications with other devices are established through the dipole antenna 12. The coaxial cable 13 is fixed at plural points by metal retainers 15.
FIG. 10 illustrates another example of this type of antenna device. As shown in FIG. 10, one antenna element of a dipole antenna 22 is formed on the front side of a dielectric plate 1, while the other antenna element is formed on the back side of the dielectric plate 1. The dipole antenna 22 is connected to a connector 24 through a feeder 23. The feeder 23 has a microstrip line configuration where conductor patterns are formed on the front and back sides of the dielectric plate 21.
FIG. 11 illustrates a still another well-known example, wherein a dipole antenna 32 formed of a conductor pattern is disposed on a dielectric plate 31, and is connected through a parallel feeder 33 to a connector 34 at an end of the dielectric plate 31. Portable radio equipment disclosed in Japanese Patent Laid-Open Publication No. 7-131221 has a configuration in which a linear antenna of a quarter wavelength and a meandering antenna element are connected through a parallel feeder to radio transmitter/receiver circuit in a housing.
There are various systems of the type having an antenna on a dielectric plate made of synthetic resin or glass to be used for communications with other devices. These systems are applicable as an antenna for the above-described portable radio equipment and as an information reading side antenna for RFID (Radio Frequency Identification Tag) systems. RFID systems are currently used in the 860-960 MHz band and in the 2.4 GHz band. If the antenna device of FIG. 9 is applied to these systems or other various radio communication systems, they need to be carefully treated during production and use. This is because the coaxial cable 13 is used as the feeder in the antenna device of FIG. 9, and the metal retainers 15 are used for fixing the coaxial cable 13 to the dielectric plate 11.
On the other hand, although the antenna device of FIG. 10 does not use retainers, the antenna device of FIG. 10 needs to have conductor patterns on both sides of the dielectric plate 21, thereby having higher production costs than antenna devices having a conductor pattern on one side. The antenna device of FIG. 11 can be produced more easily than the antenna devices of FIGS. 9 and 10, because the dipole antenna 32 and the feeder 33 are formed of conductor patterns and are disposed only on one side of the dielectric plate 31. However, in the antenna device of FIG. 11 and the above-described portable radio equipment, the parallel feeder has one bending point, and therefore there is a difference in length of a pair of lines of the parallel feeder, resulting in lowering of antenna properties.